This invention relates to an electrode interconnection material, semiconductor device using this material and driving circuit substrate for display device.
Recently, attention has been focused on an active matrix type liquid crystal display device in which a thin-film transistor (TFT) using an amorphous silicon (a-Si) film is employed as a switching element. This is because, if a TFT array is formed using an amorphous glass substrate and a-Si film producible at a low temperature, it is possible to implement an inexpensive panel display device (flat type television) of a larger screen, high quality and high definition.
Where an inverted staggered TFT is adopted having, for example, a glass substrate, gate electrode interconnection on the substrate and insulating film and a-Si film on the gate electrode interconnection, since the thin semiconductor film and data line are formed on the gate electrode and address line irrespective of their restricted thickness, it is necessary to form a thin electrode interconnection of an adequately small thickness. In the formation of a multi-layered structure an underlying electrode interconnection is tapered at the stepped edge to prevent a breakage of the overlying layer. For this reason, the following requirements are needed: for example, the workability; formability of a stable oxide film as a gate insulating film; and resistance to, for example, a sulfuric acid and hydrogen peroxide at a subsequent washing step. Conventionally, as a gate electrode interconnection material which satisfies the aforementioned requirements, use has been made of various metal films, such as tantalum and titanium. In order to attain a larger image screen and high definition, a material is desired which has a smaller electrical resistance, better workability and excellent resistance to chemicals at the subsequent step. These characteristics are required for source and drain electrode interconnection materials if a staggered TFT structure is adopted in which source and drain interconnections are formed on a substrate. A similar problem also arises from a liquid crystal display device which is not of an active matrix type.
In order to obtain a greater image screen on an active matrix type display device using the smallest possible display pixels, fine and longer gate and data lines are necessary as signal lines to TFT. Furthermore, the resistance must be made adequately smaller so as to eliminate a waveform deformation resulting from the delay of a pulse signal.
Where an active matrix type liquid crystal display device of a greater image screen and high definition is achieved, use is made of a much greater number of thin film transistors. In an array of 400 address lines.times.400 data lines, for example, 160000 pixels are required. It is difficult to completely manufacture so many thin-film transistors in an array, involving various faults, such as a short-circuiting among the interconnection layers in a multi-layered film structure, short-circuiting of a capacitor, open-circuiting of the interconnection and faults of the thin-film transistor. If a point fault is allowed for the display device, it is possible to readily remedy the open-circuiting of the interconnection layers. That is, even if the address line is broken, it can be remedied by supplying a signal from each end of the address line. The short-circuiting of the capacitor which is stored with a signal voltage can be avoided because, if the OFF resistance of thin-film transistor is made enough great and if the resistivity of the liquid crystal is made great, it is not necessary to provide such a capacitor.
The short-circuiting between the interconnection layers causes a fatal defect and if, for example, a short-circuiting occurs between the address and data lines a line fault occurs along the interconnection layer, failing to readily remedy such defect.
As a method for preventing a short-circuiting between the layers in the multi-layered structure, a multi-layered insulating film structure has been proposed in Japanese Patent Publication (Kokoku) No. 60-54478 which forms address lines and gate electrode of tantalum, anodically oxidizes the surface of the resultant structure and deposits an SiO.sub.2 or Si.sub.3 N.sub.4 film. According to this method the resistance of the address lines is increased due to the anodic oxidation of tantalum. In a thin-film transistor structure of, for example, 220.times.240 pixels for a 44 mm.times.60 mm image screen, if a 150 nm-thick address line of tantalum with a line resistance of about 60K.OMEGA. is oxidized down to about 700 .ANG., then the line resistance become about 110K.OMEGA.. For an increasing line resistance a waveform distortion becomes greater due to a delay of an address pulse signal. As a result, a time discrepancy occurs upon the writing of data into the input terminal and final end of the address line, thus impairing the uniformity of an image quality. If the thickness of the tantalum film is increased, then the line resistance can be reduced, but the tantalum film is peeled for too thick a tantalum film, providing a cause for the open-circuiting of the data line.
Molybdenum is known as a material whose resistance is smaller than tantalum. Since, therefore, molybdenum has a poor resistance to chemicals and cannot be washed in a mixture solution of a sulfuric acid and hydrogen peroxide and since a better insulating film cannot be formed on the surface, no adequate characteristic is provided which is required for the address lines at the active matrix substrate.
A similar problem also occurs in a semiconductor integrated circuit using a single crystalline silicon substrate. For example, a memory integrated circuit typically represented by, for example, a dynamic RAM is more and more integrated in its packing density. As a gate electrode interconnection of a MOS transistor which has heretofore been used for the memory integrated circuit use has usually been made of an impurity-doped polysilicon. However, too great a resistivity is involved for the polysilicon in the implementation of the microminiaturization and high integration density of the device or element. For example, molybdenum silicide (MoSi.sub.2) is known as a material which is smaller in resistivity than the polysilicon. If a dynamic RAM of about 1M bit is to be implemented using such a material, then an increase in dissipation power, signal delay, noise problem, etc. occurs due to the resistance of the electrode interconnection.